From patent DE 42 20 265 C1 a process of this type in known in which the waste materials pass sequentially through a drying zone, a carbonizing zone, a reduction zone and an oxidation zone, in order to achieve combustion and coking toward the end of the grate. In this process large proportions of slag are formed, which must be broken up by crushing rollers of correspondingly large dimensions. The energy needed for the gasification process is conveyed to the waste material through convection and radiation by the combustion which occurs at the end of the grate, via the combustion air which is introduced for the most part above the grate. This requires an especially strong combustion process at the end of the grate, with the corresponding formation of coke and slag.
The task of the invention is to achieve a stable gasification process at lower temperatures, and thereby a good burnout of residual material, while avoiding pyrolysis coke in the slag and avoiding formation of large quantities of slag.
This problem is solved in the invention, starting from a process of the type explained at the beginning, by having an arrangement of undergrate forced-draft chambers, at least in the longitudinal direction of the grate, charged with combustion air; by the fact that the waste materials are ignited in the charging area of the grate with superstoichiometric introduction of oxygen; and that in the direction of the slag removal the combustion is limited to a substoichiometric level, which is required for gasification of the combustible components.
In this process the energy needed for gasification is not introduced from outside through excessively vigorous combustion at the end of the firing grate, and hence through heating of the combustion air and through radiant energy from the roof area which has been heated up by this incineration; instead this gasification energy comes from the incineration of the waste materials, which are continuously ignited in the charging area of the grate under superstoichiometric conditions and then continue to be incinerated under substoichiometric conditions. As a result there is always an adequate supply of energy available, making possible a stable combustion process which leads to good ash burnout and to avoidance of pyrolysis coke in the left-over residue. Since the necessary energy comes from the fuel as a result of its ignition, and does not need to be transferred to the waste materials from outside, this yields a greater degree of efficiency, with the result that it is no longer necessary to burn so much of the waste material to generate energy for the gasification process, so that a larger quantity of usable gas can be created.
These beneficial effects are still further enhanced, under a refinement of the invention, by adding oxygen to the gasification air at a substoichiometric level after the waste materials are ignited.
The advantage of this measure is that the quantity of gasification air, and hence the quantity of nitrogen which functions as ballast, can be reduced very significantly, which leads to a number of benefits. One of the benefits is that as a result of the smaller quantity of gasification air the flow speed through the bed of the fuel mass is lowered, which leads to a noticeable reduction in the flow of airborne dust. An additional substantial advantage is that lowering the proportion of nitrogen reduces the formation of nitrogen oxides. There continues to be a relatively large measure of energy available for the gasification, since it is no longer necessary to heat up so much nitrogen, which is present in the gasification air only as ballast.
The gasification is preferably carried out in such a way that the gasification temperature of the substances to be gasified is 600 to 850.degree. C., and the air ratio needed to achieve substoichiometric gasification is 0.4 to 0.8.
The gases which are produced can be used in a great variety of ways. Preferably, provision is made to use the created gases by burning exclusively the combustible components which come from the first furnace in a second furnace which is connected to the first either directly or by a gas extraction system. This procedure depends primarily upon the oxygen content and the calorific value of the gases coming from the first furnace.
If the oxygen content of the waste gases coming from the first furnace is not sufficiently high, in order to achieve complete combustion it is beneficial to add combustion air in the form of ambient air to the waste gas coming from the first furnace.
On the other hand, if the calorific value of these gases is not sufficient to enable the necessary incineration at a higher temperature, then in a refinement of the invention a higher-grade combustible gas can be added to the combustible components in the second furnace, in accordance with their calorific value, along with the combustion air.
To achieve higher combustion temperatures in the second furnace than is possible with ambient air, there is benefit in mixing oxygen with the combustion air for the second furnace and/or to the higher-grade combustible gas, in a refinement of the invention. The introduction of oxygen in addition to the combustion air has the advantage that the quantity of combustion air which needs to be added can be kept down, which allows the quantity of waste gas from the second furnace to be reduced.
If there is sufficient oxygen present in the waste gas coming from the second furnace, and if the calorific value is also sufficiently high, waste gas which is taken from the waste gas stream after cooling in a waste heat recovery system can be recirculated into the second furnace to mix with the gas entering the second furnace from the first furnace. This burning of the mixed gases helps to regulate the combustion in the second furnace order to regulate the combustion in the second furnace. This recirculation procedure has the advantage that any gases which may still be combustible when they leave the second furnace are more completely burned during one or more additional traverses through the second furnace. Moreover, with this recirculation procedure the quantity of waste gas is lower compared to other procedures in which a higher grade combustible gas is added to the second furnace.
The incineration in the second furnace is carried out by selecting from the measures described above, preferably in such a way that in the second furnace the air ratio is 1.1 to 1.8. The combustion temperature of the gases in the second furnace is 950 to 1250.degree. C.
The device for carrying out the process is characterized by the fact that in a furnace which comprises a fuel charging system, a grate with undergrate forced draft chambers subdivided in the longitudinal direction and possibly also in the transverse direction, and a slag removal system, the forced draft chambers have lines for the introduction of oxygen. This makes it possible to regulate the gasification with a fine touch, and furthermore with smaller quantities of waste gas compared to the exclusive use of ambient air for the gasification process.
If utilization of the generated gases by burning is planned, then the invention provides for connection of a second furnace with the first furnace, either directly or through a waste gas flue. This prevents any significant cooling of the combustible gases which arise in the first furnace, and consequently the combustion in the second furnace can be carried out at a relatively low excess air ratio, in many cases even without additional sources of energy, at such high temperatures that all of the organic components in the combustible gas can be broken down without promoting the formation of nitrogen oxides, since the temperature in the second furnace is kept at a level which is lower than a critical temperature limit at which greater quantities of nitrogen oxides begin to be formed.
In order to be able to carry out the advantageous refinements of the process, advanced implementations of the device provide for the second furnace to be equipped with at least one line for the introduction of combustion air, at least one line for the introduction of a higher-grade combustible gas, or at lease one line for the introduction of recirculated waste gas. These three measures can optionally be provided individually or in combination, depending upon the type of implementation.